1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser in which a longitudinal mode is controlled by a diffraction grating in a laser in order to obtain a dynamic single mode oscillation.
2. Description of the Related Art
Mode control of a semiconductor laser is an important factor for determining laser characteristics. A laser which provides stable oscillation characteristics at a single mode finds an increasing demand in optical communication and other fields, and intense studies have been made to develop such a laser.
In general there are three types of modes in semiconductor lasers: namely, vertical transverse mode, horizontal transverse mode and longitudinal mode. In recent years, various laser structures have been proposed in which vertical and horizontal transverse modes are well controlled.
On the other hand, the longitudinal mode has been controlled mainly by Fabry-Perot type resonator which makes use of the end surfaces of the laser cavity as reflection surfaces. This control method, however, tends to cause a multi-mode oscillation, i.e., oscillations at multiple wavelengths, particularly when high-speed modulation is performed. This has given a rise to the demand for a stable control method for realizing single mode oscillation.
To meet this demand, a laser device has been proposed in which a diffraction grating is provided in the laser to serve as a resonator. In particular, a semiconductor laser of the type referred to as a "distribution feed back" (DFB) type laser is being considered as promising. In this type of laser only a specific wavelength component is reflected or fed back by a diffraction grating arranged within a gain region.
In a known DFB type semiconductor laser, a diffraction grating is formed by providing convex and concave layers adjacent to a light-emitting laser-active layer. That is, regions of different crystalline structures are arranged periodically along the optical (light emitting) axis in the vicinity of the active layer with the borders constituted by the concavities and convexities, so that the light propagating from the active layer undergoes a periodic change in the refractive index through these regions.
This known arrangement, however, is disadvantageous in that the diffraction efficiency is small because the diffraction grating is formed in layers other than those that form the laser-active layer.